Many metal alloys are made by mixing raw metals in selected proportions in a furnace and heating the mixture at a desired temperature for a selected time to melt the metals. In many processes the molten metal is taken from the furnace in ladles. Other additives may be added to the molten metal while the melt is in the ladle. Then the metal is poured from the ladle into molds. These molds are often made of cast iron or another metal having a higher melting point than the metal being melted. Those temperatures are often in the range of 2750° F. to 2800° F. (1510° C. to 1538° C.). The molten metal solidifies in the mold to form an ingot, electrode or other metal product. The metal product is removed from the mold for further processing. The mold is cleaned and then reused to form other metal products.
Electrodes for arc welding and electric arc furnaces are commonly made by pouring molten metal into a cast iron tubular body that sits on a flat base. The molten metal solidifies inside that tubular body forming an electrode and then the tubular body is lifted off the base. Sometimes the electrode remains on the base when the tubular body is removed. At other times the electrode stays within the tubular body and must be removed from that body.
When the electrode is removed from a tubular body or base, pieces of metal will remain in the tubular body, stuck to the bottom or sides, or on the base. These pieces of metal must be removed before the tubular body and base can be reused. Depending upon the size of the electrode as much as 75 pounds (34 kg) of metal may be left behind and becomes scrap.
The pouring of hot metal into the electrode mold and the removal of the electrode causes wear on the mold. Consequently, electrode molds must be replaced or refurbished. Cast iron electrode molds have a life of about twenty heats. There has been a long, unsatisfied need for electrode molds, as well as other molds used to create other metal parts, which have a longer service life and which produce less scrap.
Molds having cavities much smaller than the cavity in an electrode mold or ingot mold are used for making molded plastic products and for casting metal parts. In the plastic molding industry there are two basic types of molds. One type of mold is created by removing material from mating mold plates such that when the two mold plates are put together the corresponding cavities in the mold plates define a single cavity that has the shape of the product to be molded. Because the mold plates are made from wear-resistant metal alloys, conventional machining of these plates is extremely difficult, expensive, time consuming, and is also limited to machining simple mold cavity geometries.
The second type of mold, known as an insert mold, has a mold body or mating mold plates which have a space that is sized to receive one or more inserts. Metal inserts are placed into the space machined into the mold plates or mold body. The inserts have cavities which together define the shape of the part to be molded. Inserts allow the use of lower cost and easier to machine metals for the mold plates and limit the use of wear-resistant metal alloys to the metal inserts. However, as with the first method, this approach requires expensive and time consuming metal removal methods to create the cavity shape within the metal insert.
It is also known in the art to use mold inserts made of a ceramic material in molds used for plastic injection molding. U.S. Pat. No. 4,704,079 to Mini Jr, discloses a multi-part mold for injection molding of plastic parts. This mold is made up of at least two parts. There is a mold block portion having a ceramic mold cavity insert. The ceramic mold cavity inserts define the walls of the mold cavity when the mold is closed. Each mold cavity insert comprises a shaped ceramic body having a mounting surface adapted to fit under compressive stress throughout the entire molding operation within a mounting cavity of the mold block portion.
Injection molding is extremely hard on molds. Although some ceramic inserts last longer than others, they all suffer eventually because of distortion, erosion of the mold cavity surface, heat checking from thermal shock, and for other reasons. In order for a ceramic insert to be utilized in a mold cavity for high pressure molding, it is necessary to secure it in a mounting cavity in the mold block or mold body portion in such a way as to maintain it in a state of compressive stress throughout the molding or casting cycle. This will provide a tight fit of the insert in the mold block despite differences in the thermal expansion and thereby provide efficient transfer of impact energy and thermal energy to the mold block. Consequently, the space that receives the ceramic insert, as well as the insert itself must be specially configured to fit together in a way that provides a tight fit.
U.S. Pat. No. 7,302,989 Kamel, et al. discloses a mold system that may include a method of producing a ceramic core usable in production of a turbine airfoil. The method may include building a mold plate having at least one mold cavity configured to receive an insert and installing an insert in that cavity. Installing the insert in the cavity preferably includes installing a ceramic insert fully or partially formed from silicon carbide. The insert may include a coating formed from a chemical vapor deposition of a nonporous material, which may be silicon carbide. The ceramic insert may be formed from a net shape process. The mold plate may be formed with soft steels, such as P20 or NAK55, aluminum, aluminum-epoxy, and other appropriate materials. Materials such as abrasive resistant steels are not preferred because such materials are difficult to machine and require EDM processes.
In the molding systems in which ceramic inserts have been used the amount of material that enters the mold cavity is usually a small fraction of the amount of molten metal that is poured in an electrode mold or ingot mold and those molds operate at much lower temperatures. Consequently, the use of ceramic inserts in molds for injection molding of plastics has not been considered as being transferrable to molds used in steel making.